Tools for the chip forming metal machining consist of a substrate body of cemented carbide, cermet, ceramics, steel or high-speed steel which in the most cases is provided with a single-layer or multi-layer hard material coating for the improvement of the cutting properties and/or the wear properties. The hard material coating consists of layers of mono-metallic or mixed-metallic hard material phases. Examples of mono-metallic hard material phases are TiN, TiC, TiCN, AlN and Al2O3. Examples of mixed-metallic phases, wherein within one crystal a metal is in part replaced by another one, are TiAlN and TiAlCN. Coatings of the afore-mentioned type are deposited by CVD processes (chemical vapor deposition), PCVD processes (plasma-enhanced CVD processes) or by PVD processes (physical vapor deposition).
In almost every material there are residual stresses due to the manufacturing process and/or of mechanical, thermal and/or chemical treatment. During the production of tools by coating of a substrate body by CVD processes, residual stresses, for example between the coating and the substrate and between the individual layers of the coating, respectively, result from the different coefficients of thermal expansion of the materials. The residual stresses can be tensile stresses or compressive stresses. In coatings deposited by PVD processes, in general, there exist residual compressive stresses due to, amongst other reasons, the ion bombardment in this process. In contrast to that, CVD processes usually generate residual tensile stresses in the coating.
There is distinguished between three types of residual stresses: macro-stresses that are almost homogenously distributed across macroscopical regions of the material, micro-stresses that are homogenous within microscopic regions, such as for example within a grain, and inhomogeneous micro-stresses that are inhomogeneous even at a microscopic level. From a practical point of view and for the mechanical properties of a cutting tool the macro-stresses are of particular importance.
The residual stresses in hard material coatings can have a significant influence on the properties of the coatings with advantageous or disadvantageous effects on the wear resistance of the tool. Residual tensile stresses exceeding the tension limit of the respective material can cause fractures and cracks in the coating perpendicular to the direction of the residual tensile stress. In general, a particular amount of residual compressive stress in the coating is desired, because it avoids and closes surface cracks and improves the fatigue properties of the coating and thereby of the tool. However, too high residual compressive stresses can lead to adhesion problems and to chipping of the coating.
To increase the residual compressive stresses in the hard material coating of a tool, especially in hard material coatings deposited in the CVD process, it is known to subject the tool to a mechanical surface treatment. Known mechanical treatment methods are the treatment by brushing and dry blasting or wet blasting. In the blasting treatment a fine grained blasting medium is directed onto the surface of the coating using compressed air under increased pressure. Such a surface treatment can reduce residual tensile stresses and increase residual compressive stresses in the outermost layer, but also in the layers of the hard material coating arranged thereunder.
Single-layer or multi-layer wear protective coatings of tools for the chip forming metal machining often comprise one or more polycrystalline TiAlN and/or TiAlCN layers having predominantly face-centered cubic crystal structure (fcc-TiAlN, fcc-TiAlCN). Especially fcc-TiAlN layers have proven of value for the wear protection of tools for multiple chip forming applications. Compared to the monometallic border systems of the hard material phases fcc-TiN, fcc-TiCN or AlN having hexagonal structure (w-AlN), fcc-TiAlN and fcc-TiAlCN layers are characterized by the advantageous combination of increased hardness and oxidation resistance. Fcc-TiAlN and fcc-TiAlCN layers are thermodynamically metastable. It is known that fcc-TiAlN deposited by PVD processes decomposes at increased temperature first by spinodal decomposition into Al-rich and Ti-rich domains of fcc-AlN and fcc-TiN, respectively. This goes along with an increase of the hardness and is known as “age hardening effect”. When the temperature is further increased, the layer decomposes into the thermodynamically stable phases w-AlN (AlN having wurtzite structure) and fcc-TiN. For example, P. H. Mayrhofer et al., Applied Physics Letters, 83/10 (2003), pages 2049-2051, report about a spinodal decomposition of a Ti0.5Al0.5N layer, which is deposited in the PVD process, at 860° C. beginning from the bulk of the coating material. The hardness of the coating decreases beginning at about 850° C., and above about 950° C. it is lower than in the non-annealed state. Adibi et al., J. Appl. Phys., 69/9 (1991), pages 6437-6450 report about the PVD deposition of Ti0.5Al0.5N coatings, wherein a surface-initiated spinodal decomposition is observed in a temperature range of 540-560° C., and at a further increase of the temperature a phase separation into fcc-TiN and w-AlN is observed. Significant proportions of w-AlN phase, which is softer than fcc-TiAlN and fcc-TiN phases, lead to unfavorable mechanical and tribological properties of the coating. The phase conversion is promoted by stacking faults.